1. Field of the Invention
This invention relates to generating electricity and more particularly relates to generating electricity from a chemical hydride.
2. Description of the Related Art
As the cost of fossil fuels increases, pollution increases, and the worldwide supply of fossil fuels decreases, alternative energy sources are becoming increasingly important. Hydrogen is a plentiful alternative energy source, but it generally exists in a combination with other elements, and not in a pure form. The additional elements add mass and may prevent the hydrogen from being used as an energy source. Pure hydrogen, however, is a desirable energy source. Pure hydrogen comprises free hydrogen atoms, or molecules comprising only hydrogen atoms. Producing pure hydrogen using conventional methods is generally cost prohibitive.
One way that pure hydrogen can be generated is by a chemical reaction which produces hydrogen molecules. The chemical reaction that occurs between water (H2O) and chemical hydrides produces pure hydrogen. Chemical hydrides are molecules comprising hydrogen and one or more alkali or alkali-earth metals. Examples of chemical hydrides include lithium hydride (LiH), lithium tetrahydridoaluminate (LiAlH4), lithium tetrahydridoborate (LiBH4), sodium hydride (NaH), sodium tetrahydridoaluminate (NaAlH4), sodium tetrahydridoborate (NaBH4), and the like. Chemical hydrides produce large quantities of pure hydrogen when reacted with water, as shown in reaction 1.NaBH4+2H2O→NaBO2+4H2  (1)
Recently, the interest in hydrogen generation has increased, because of the development of lightweight, compact Proton Exchange Membrane (PEM) fuel cells. One by-product of generating electricity with a PEM fuel cell is water, which can be used or reused to produce pure hydrogen from chemical hydrides for fuelling the PEM fuel cell. The combination of PEM fuel cells with a chemical hydride hydrogen generator offers advantages over other energy storage devices in terms of gravimetric and volumetric energy density.
Unfortunately, the prior art has encountered unresolved problems producing pure hydrogen from chemical water/hydride reactions. Specifically, conventional systems, methods, and apparatuses have not successfully controlled the chemical reaction between the water and the chemical hydride without adversely affecting the gravimetric and volumetric energy density of the overall system. This lack of control also prevents conventional systems, methods, and apparatuses from meeting dynamic hydrogen demands of PEM fuel cells.
The chemical reaction between water and a chemical hydride is very severe and highly exothermic. The combination of water and a chemical hydride must be precisely controlled to prevent a runaway reaction or an explosion. Many failed attempts have been made to properly control the reaction while still preserving the gravimetric and volumetric energy density provided by the chemical hydrides
One attempt to properly control the reaction involves separating water from the chemical hydride using a membrane. Generally, the membrane passes water because of a difference in water pressure across the membrane. Water pressure on the side of the membrane opposite the chemical hydride pushes water through the membrane, because water is quickly used up in the reaction with the chemical hydride. Other membranes utilize a capillary action to transport water from one side of the membrane to the other. Consequently, a water supply must be provided that supplies water to the water side of the membrane to be transported by capillary action to the chemical hydride side of the membrane. Because the reaction is membrane controlled, it is difficult to dynamically increase or decrease hydrogen production based on the demands of a PEM fuel cell or other hydrogen consuming device.
Another side effect of such a system is that the chemical hydride will “gum” or “clump” as water is introduced. Gumming or clumping refers to the spheres or other geometric shapes formed by the chemical hydride and its byproducts during the reaction. Water is able to react with the outer portion of the “clump” to a certain depth, however, large portions of the “clump” remain unreacted because water cannot penetrate deeply enough into the “clump.” Consequently, the gravimetric and volumetric energy density is decreased because of the large percentage of the chemical hydride that remains unreacted. This is inefficient and greatly increases the amount of the chemical hydride that such systems use to create a given amount of hydrogen.
Accordingly, what is needed is an improved apparatus, system, and method that overcome the problems and disadvantages of the prior art. The apparatus, system, and method should promote a substantially complete reaction of a chemical hydride reactant. In particular, the apparatus, system, and method should be dynamically controllable to satisfy the varying hydrogen requirements of generating electricity.